Although heat resistant steels are used in various areas, materials of turbines and boilers are the typical uses of the ferritic heat resistant steels. Therefore, the heat resistant steels of this invention will be specified in terms of turbine and boiler materials hereinafter.
Most of conventional heat resistant steels hitherto developed for use in boiler and turbine materials contained 9 to 12% chromium as well as one or more of carbon, silicon, manganese, nickel, molybdenum, tungsten, vanadium, niobium, titanium, boron, nitrogen and copper, in amounts of 0.04 to 2.0%, respectively. It should be noted that "percent (%)" means "mass %" herein unless any explanatory note is given.
Compositions of typical heat resistant steels for materials of turbines and boilers are listed in Table 1 and Table 2 (refer to "Compositions, Structures and Creep Characteristics of Heat Resistant Alloys" distributed as a brief at the 78th conference held under co-sponsorship of Japan Metal Society and Kyushu branch of Japan Iron and Steel Institute . . . Reference 1). All these steels have been developed by many experiments wherein various elements of various amounts were alloyed in turn. The action and function of each said alloying element has come to be known by such trial-and-error experiments and can be roughly summarized as follows.
Chromium
Chromium improves corrosion and heat resistance of the steel. Chromium content should be increased as the service temperature of the steel is elevated.
Tungsten, Molybdenum
These elements improve high temperature strength of the steel due to their function for bringing about solid solution hardening and precipitation hardening in the structure of the steel. However, as contents of these elements are increased, the ductile-brittle transition temperature (DBTT) of the resultant steel is elevated. In order to suppress the embrittlement of the steel, the molybdenum equivalent [Mo+(1/2)W] is necessarily lowered below 1.5%. In accordance with this instruction, the molybdenum equivalent of most of the conventional alloys is around 1.5%.
Vanadium, Niobium
These elements will bring about strengthening of a steel due to formation of carbo-nitrides through precipitation hardening. The solid solubility of vanadium in a steel is 0.2%, whereas that of niobium is 0.03%, when the steel is annealed at a temperature of 1050.degree. C. If the amount of vanadium and that of niobium exceed their respective solid solubility, the excess amount of vanadium and that of niobium will form their carbides and nitrides in the steel matrix during annealing. Results of experimental work obtained up to the present, in particular that of creep rupture tests, show that the optimum vanadium and niobium contents are 0.2% and 0.05%, respectively. The niobium content "0.05%" in the steel exceeds its solid solubility, and the excess niobium forms NbC which is effective to suppress coarsening of austenitic crystal grains during annealing heat treatment.
Copper
As copper is one of the austenite stabilizing elements, it suppresses formation of the .delta. -ferrite as well as precipitation of iron carbides. Copper in the steel exhibits a weak action of lowering the Ac.sub.1 point and improves hardenability of the steel. Copper suppresses forming a softened layer in a heat affected zone (hereinafter designated as HAZ). However, addition of more than 1% copper to a steel decreases its reduction of area upon creep rupture.
Carbon, Nitrogen
These elements are effective to control structure and strength of the steel. Concerning creep properties of the steel, the optimum carbon and nitrogen amounts for creep rupture strength depend on contents of vanadium, niobium or the like carbide and/or nitride forming elements in the steel.
Boron
About 0.005% of boron in a steel improves its hardenability. It is said that boron is further effective to make the steel structure fine and thereby to improve strength and toughness.
Silicon, Phosphorus, Sulphur, Manganese
In order to suppress embrittlement of the steel by making it super-clean, these elements are desired to be as low as possible. However, silicon has an effect of suppressing oxidizing attack of water vapor on the steel. So it is said that some amount of silicon should be kept in the boiler steel.
The action and function of each alloying element are clarified to some extent in accordance with the conventional alloy developing method, as mentioned above. However, a great deal of experimental work will be required before obtaining a novel sort of steel with desirable chemical and physical properties. For example, in a steel containing five alloying elements, if the content of each element is changed in three content levels, 3.sup.5 combinations could be produced and such huge numbers of alloys have to be melted, cast and formed into various test specimens, followed by a great deal of experimentations.
As shown in Tables 1 and 2, most of the heat resistant steels recently developed contain more than ten alloying elements. Development of new steels like the steels in FIGS. 1 and 2 in accordance with the conventional trial-and-error method requires a great deal of labor, time and cost.
We, the inventors, already developed a method of designing novel metallic materials on the basis of a molecular orbital theory. An outline of the method is disclosed in "Journal of Metal Institute of Japan, Vol.31, No.7(1992), pp 599-603" (Reference 2) and "Altopia, September 1991, pp. 23-31" (Reference 3). Meanwhile, we filed a Japanese Patent Application relating to "A Method of Producing Nickel Base Alloys and Austenitic Ferrous Alloys" [refer to Japanese Patent No.1,831,647 (Japanese Patent Publication No.5-40806) corresponding to U.S. Pat. No. 4,824,637].
It is certain that, in view of the above-mentioned references and patent documents, the novel alloy designing method is applicable to produce aluminum base alloys, titanium base alloys, nickel base alloys and the like nonferrous alloys, intermetallic compound alloys and austenitic iron-base alloys. However, it has not been certain that the novel alloy designing system can be applicable to produce ferritic heat resistant steels.
This invention has been accomplished to provide a novel alloy designing system for producing iron base alloys, particularly ferritic heat resistant steels, without the need of troublesome trial-and-error experimentation.
Therefore, an object of this invention is to provide a method of producing with high efficiency ferritic iron base alloys excellent in high temperature strength on the basis of theoretical predicting system.
Another object of this invention is to provide ferritic heat resistant steels which are excellent in various physical and chemical properties such as high temperature strength, as compared with the conventional ferritic heat resistant steel and therefore are well applicable to turbine and boiler materials which are durable even for a severe water vapor environment of 246-351 kgf/cm.sup.2 g pressure and 538-649.degree. C. temperature.